1. Field of the Invention
The invention relates to a developing chamber for electrostatic latent images which is provided with a device by which a developer consisting of electroscopic particles and a carrier material can be applied in the field of a magnet to a record carrier passed through the chamber, and can be carried along by the record carrier owing to frictional forces. The developer required is a mixture of fine electroscopic particles, frequently referred to as toner, and a granular carrier material. The two materials are charged triboelectrically and hence adhere to one another. When this developer comes into contact with the record carrier provided with the latent image, the carrier grains give up the toner to the record carrier owing to the electrostatic force of attraction. The carrier grains themselves return to the developing chamber, where they are recharged with toner. The developer may be supplied to the record carrier, for example, by the cascade method or, in the case of ferromagnetic carrier materials, by the turbulence chamber method.
2. Description of the Prior Art
In the cascade method, the developer is conveyed to an elevated part of the record carrier and then flows down over the surface of the record carrier after the fashion of a waterfall to the bottom of the chamber and subsequently is conveyed upward again. In order to intensify development of the latent image, particularly if large areas are to be developed, additional baffle plates may be inserted into the cascade flow by which the developer is pressed against the record carrier. The record carrier travels in the direction in which the developer is conveyed. If in addition the baffle plate is connected to a voltage source, the pressure on the record carrier and hence the intensity of image development can be increased by the ensuing potential drop. An arrangement of this type is described, for example, in German Offenlegungsschrift No. 2,118,629.
In the turbulence chamber method the developer, which in this case consists of a toner and powdered iron, is attracted in the field of a magnet disposed behind the record carrier which travels in a substantially vertical direction and is carried along by the record carrier owing to frictional forces until it drops down again at the upper edge of the magnetic field (German Offenlegungsschrift No. 1,522,620). Circulation of the developer in such a "turbulence eddy" requires no external driving elements except those for paper transport. Because the developer owing to its content of metal carrier material is electrically conductive, it can simultaneously perform the function of the auxiliary electrode referred to in the above-described cascade method. This method has the advantage that the developer is in contact with the record carrier, while in the cascade method the effect of the auxiliary electrodes is adversely affected by the larger distance.
With these known apparatus the conductivity of the loosely cohering developer mixture in the turbulence chamber, particularly in the case of comparatively high concentrations of toner, is comparatively low so that the uneven transport of charged toner particles by the record carrier may give rise to undefined potential conditions in the layer of developer in contact with the record carrier.
In the chambers described above the developer is carried along by the paper (the record carrier) without any slip, in contrast with, for example, what is generally referred to as the magnetic brush method in which the developer rubs on the paper. However, a certain degree of friction promotes intimate contact between the developer and the record carrier and improves development of the charge image.